public class Keye04{
	
	public static void main(String[] args){
		int i = 0;
		int sum = 0;
		int a = 0;
		int b = 0;
		while(i<=100){
			if(i%2 ==0){
			a += i;
			}else {
			b += i;	
			}
			i++;
			sum =a-b+1;
		}
		System.out.println(sum);
	}
	
}		